Medical implants have been in use in medicine for a long time aiming to support or substitute for body functions. In particular defective joints of the body are replaced by articular implants with increasing success which often is associated with a substantial improvement in the quality of life of the respective patient.
Aside from cemented articular endoprostheses, non-cemented articular endoprostheses are also in use currently for replacement of hip, knee, and shoulder joints. Said non-cemented articular endoprostheses are generally made of titanium alloys and usually have a roughened (often sand-blasted) or textured porous surface in order to improve the integration of the bone tissue.
The prior art includes attempts aiming to improve the compatibility of the implant surfaces with respect to the bone tissue. Invariably, a load-bearing bulk phase is to contribute most of the requisite biomechanical properties (structural compatibility) and the surface phase is to ensure the compatibility (surface compatibility) to the adjoining bone tissue (WINTERMANTEL: Biokompatible Werkstoffe und Bauweisen: Implantate für die Medizin und Umwelt. Springer Verlag, Berlin, Heidelberg, New York, 1996). The underlying concept was to generate surface textures which would simulate the mineral phase of bone tissue. The mineral phase of human bone tissue is formed by a carbonate-apatite/hydroxyapatite. For this reason, the emphasis was on the development of calcium phosphate layers to improve the surface compatibility of implants.
Various methods (thermal spraying methods, electrochemical deposition, sol-gel technologies, ion beam sputtering, laser ablation) have been used in regions contacting the bone tissue (articular endoprostheses of hip, knee, shoulder) attempting to improve the surface compatibility. Thus far, only plasma spraying (DE GROOT, KLEIN, WOLKE: Plasma-sprayed coatings of calcium phosphate. CRC Press, Boca Raton, Ann Arbor, Boston, 1990; DE GROOT, KLEIN, WOLKE: Chemistry of calcium phosphate bioceramics. CRC Handbook of bioactive ceramics, 2 (1996) 3-16; WO2009062671) and electrochemical deposition of calcium phosphate layers (BAN, MARUNO: Morphology and microstructure of electrochemically deposited calcium phosphates in a modified simulated body fluid. Biomaterials, 19 (1998) 1245-1253; DE4431862; WO2009147045; CN101485901; CN101406711; EP2037980; US2006134160; WO2004098436; WO2004024201; EP1264606; EP0774982; EP0232791) have become established on an industrial scale.
However, clinical long-term studies showed that plasma-sprayed calcium phosphate layers, initially considered to be stable in the long term, are subject to partial degradation in their biological environment (WHEELER: Eight-year clinical retrospective study of titanium plasma-sprayed and hydroxyapatite-coated cylinder implants. International Journal of Oral and Maxillofacial Implants, 11,3 (1996) 340-350. OSHBORN: Die biologische Leistung der Hydroxylapatitkeramik-Beschichtung auf dem Femurschaft einer Titanendoprothese-erste histologische Auswertung eines Humanexplantats. Biomedizinische Technik, 32 (1987) 177-183). During this process, there are phase changes at the interface to the bone tissue and the process leads to encapsulation and/or flaking mainly of crystalline components of the layer (particles).
Studies by Cooley (COOLEY, VAN DELLEN, BURGESS, WINDELER: The advantages of coated titanium implants prepared by radiofrequency sputtering from hydroxyapatite. J. Prosthet. Dent., 67 (1992) 93-100.) and Maxian (MAXIAN, ZAWADSKI, DUNN: Effect of CaP coating resorption and surgical fit on the bone/implant interface. Journal of Biomedical Material Research, 28 (1994) 1311-1319.) demonstrated the high efficiency of fully degradable, bioactive layers that were applied to metallic base bodies by means of electrochemical procedures. From the analysis of animal experiments and clinical studies, it was concluded that safe osseointegration at the implant surface is evident despite the rapid and complete degradation of highly soluble calcium phosphate layers.
It can therefore be noted that the rapidly soluble calcium phosphate layers can afford good clinical outcomes. Obviously, improved compatibility of implant surfaces with respect to the bone tissue does not require a coating on the implant surfaces that is stable in the long term.
However, like with all electrochemical deposition procedures common thus far, it is disadvantageous that a substantial equipment and time effort is required in order to apply these calcium phosphate layers to these medical implants.
Accordingly, there is a need for a simple, inexpensive, and rapidly applicable method that allows the surfaces of medical implants to be provided with coatings that promote bone growth.